1. Technical Field
The present invention relates to a tire state monitoring apparatus that monitors the state of tires, and in particular, to a tire state monitoring apparatus that can monitor a state, such as the air pressure of tires used for automobiles, etc.
2. Description of the Related Art
Conventionally, in order to enhance the safety during traveling in a vehicle, such as automobiles, tire state monitoring apparatuses that monitor a state, such as the air pressure of each of the tires have been developed. For example, a tire state monitoring apparatus in which four tires of a vehicle are provided with transponders, respectively, four antennas corresponding to the transponders and one transceiver connected to these antennas are provided on the vehicle side, and the number of times of transmitting an address electric wave to each of the transponders is increased or reduced according to the speed of the vehicle is suggested for example, in U.S. Pat. No. 6,931,923. In the tire state monitoring apparatus disclosed in U.S. Pat. No. 6,931,923, an angle within which transmission and reception can be reliably made between the antennas of the transceivers and the transponders is limited, and the angle is a total of 90° including 45° on the right and left sides towards an antenna of the transceiver from the center of a tire. Thus, the number of times of transmitting an address electric wave to a transponder is increased or reduced so that transmission and reception can be made within this angle. According to this tire state monitoring apparatus, it is possible to reliably monitor the state of tires irrespective of the speed of vehicle.
In a conventional tire state monitoring apparatus disclosed in U.S. Pat. No. 6,931,923, as a basic technical idea, the time during which transmission and reception can be made according to the speed of a vehicle is determined from the angle within which transmission and reception can be made, this time is divided by the time that is taken for a tire to make one rotation, and the number of times of wireless transmission while the tire makes one rotation is determined. If the number of times is not an integer, it is carried and obtained as an integer. Meanwhile, in such a conventional technique, the number of times obtained becomes an integer according to a vehicle speed, without being carried. In this case, the interval time of transmission according to the number of times obtained will coincide with the time during which transmission and reception can be made. In this case, although rare, the boundary of the above angle within transmission and reception can be made, and the timing of transmission might be synchronized with each other. Although depending on the determination reference of the angle within which transmission and reception can be made, when it is considered that the number of times of transmission is made as small as possible, this angle becomes wide over the ability of communication sensitivity of an apparatus. In that case, since transmission and reception becomes unstable in a boundary portion of the angle, the aforementioned synchronization phenomenon will hinder stable transmission and reception. Further, if the angle is set narrow over the ability of an apparatus in order to ensure transmission and reception, the number of times of transmission increases. As a result, it is necessary to enhance the communication performance of an apparatus. Even if the number of times of transmission does not become an integer, the same phenomenon occurs with some difference in degree if the number of times of communication is close to an integer. In the above conventional technique, since the number of times of transmission in one rotation of a tire increases if the speed of a vehicle becomes high, the transmission intervals become short beyond the communication ability of an apparatus. Therefore, if the speed of the vehicle exceeds 100 km/h, a separate method is adopted.
As described above, the conventional tire state monitoring apparatus has a disadvantage in that the transmission interval and the angle within which transmission and reception can be made is synchronized with each other depending on the speed of a vehicle. Further, since the number of times of transmission while the tire makes one rotation is increased by shortening the time intervals at which address electric waves are transmitted as the speed of the vehicle becomes high, a processor capable of coping with high-speed processing according to the maximum number of times of the address electric waves should be built in. As a result, there is the disadvantage that the cost required for manufacturing such an apparatus becomes high.